EP4306231A1 - Thermal processing method for niobium-containing high-alloy austenitic heat-resistant stainless steel bar - Google Patents
Thermal processing method for niobium-containing high-alloy austenitic heat-resistant stainless steel bar Download PDFInfo
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- EP4306231A1 EP4306231A1 EP22841031.2A EP22841031A EP4306231A1 EP 4306231 A1 EP4306231 A1 EP 4306231A1 EP 22841031 A EP22841031 A EP 22841031A EP 4306231 A1 EP4306231 A1 EP 4306231A1
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- 229910001220 stainless steel Inorganic materials 0.000 title claims abstract description 25
- 239000010935 stainless steel Substances 0.000 title claims abstract description 25
- 229910052758 niobium Inorganic materials 0.000 title claims abstract description 17
- 239000010955 niobium Substances 0.000 title abstract description 34
- 229910045601 alloy Inorganic materials 0.000 title abstract description 14
- 239000000956 alloy Substances 0.000 title abstract description 14
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 title abstract description 13
- 238000003672 processing method Methods 0.000 title abstract description 5
- 238000005242 forging Methods 0.000 claims abstract description 53
- 238000000034 method Methods 0.000 claims abstract description 36
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 31
- 238000005098 hot rolling Methods 0.000 claims abstract description 31
- 239000010959 steel Substances 0.000 claims abstract description 31
- 238000010438 heat treatment Methods 0.000 claims abstract description 30
- 230000006835 compression Effects 0.000 claims abstract description 4
- 238000007906 compression Methods 0.000 claims abstract description 4
- 238000005096 rolling process Methods 0.000 claims description 11
- 229910052804 chromium Inorganic materials 0.000 claims description 5
- 229910052759 nickel Inorganic materials 0.000 claims description 5
- 229910052748 manganese Inorganic materials 0.000 claims description 4
- 239000000203 mixture Substances 0.000 claims description 4
- 230000008569 process Effects 0.000 abstract description 17
- 238000004519 manufacturing process Methods 0.000 abstract description 10
- 230000007547 defect Effects 0.000 abstract description 2
- 239000000463 material Substances 0.000 description 12
- 238000007689 inspection Methods 0.000 description 7
- 229910000963 austenitic stainless steel Inorganic materials 0.000 description 5
- 238000005336 cracking Methods 0.000 description 4
- 230000000694 effects Effects 0.000 description 4
- 238000007796 conventional method Methods 0.000 description 3
- 238000000227 grinding Methods 0.000 description 3
- 239000002184 metal Substances 0.000 description 3
- 229910052751 metal Inorganic materials 0.000 description 3
- 238000001953 recrystallisation Methods 0.000 description 3
- 238000011160 research Methods 0.000 description 3
- 238000007711 solidification Methods 0.000 description 3
- 230000008023 solidification Effects 0.000 description 3
- 238000004090 dissolution Methods 0.000 description 2
- 230000005496 eutectics Effects 0.000 description 2
- 238000010304 firing Methods 0.000 description 2
- 230000008570 general process Effects 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 239000002105 nanoparticle Substances 0.000 description 2
- 150000002822 niobium compounds Chemical class 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- 238000007655 standard test method Methods 0.000 description 2
- 229920001169 thermoplastic Polymers 0.000 description 2
- 239000004416 thermosoftening plastic Substances 0.000 description 2
- 238000005275 alloying Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 238000001192 hot extrusion Methods 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 238000010921 in-depth analysis Methods 0.000 description 1
- 239000011159 matrix material Substances 0.000 description 1
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- 239000004033 plastic Substances 0.000 description 1
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- 238000005728 strengthening Methods 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 239000002699 waste material Substances 0.000 description 1
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Classifications
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
- B21C37/04—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of bars or wire
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/16—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling wire rods, bars, merchant bars, rounds wire or material of like small cross-section
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
- B21B3/02—Rolling special iron alloys, e.g. stainless steel
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
- B21J5/00—Methods for forging, hammering, or pressing; Special equipment or accessories therefor
- B21J5/002—Hybrid process, e.g. forging following casting
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
- C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
Definitions
- the present disclosure relates to the technical field of steel material processing, and in particular, to a hot working method of a niobium-containing high-alloy austenitic heat-resistant stainless steel bar.
- the present disclosure is proposed in order to provide a hot working method of a niobium-containing high-alloy austenitic heat-resistant stainless steel bar that overcomes the above problems or at least partially solves the above problems.
- a hot working method for a stainless steel bar comprising:
- a surface temperature at the beginning of the radial forging is in a range from 1,000 °C to 1,050 °C, and a surface temperature during the radial forging and at the end of the radial forging is in a range from 900 °C to 950 °C.
- step (2) the radial forging is carried out in 4 to 10 passes, a single pass deformation and a forging frequency for the first and second passes are respectively 4% to 8% and 200 to 240 times/min, and a single pass deformation and a forging frequency for the subsequent passes are respectively 15% to 20% and 30 to 50 times/min.
- a surface temperature at the beginning of the hot rolling is in a range from 1,100 °C to 1,150 °C, and a surface temperature during the hot rolling and at the end of the hot rolling 1,000 °C or more.
- step (4) the hot rolling is carried out in 5 to 10 passes, a single pass deformation is 10% ⁇ 15%, and a rolling speed is 1 ⁇ 1.2m/s.
- the stainless steel bar has the following elemental composition:
- a stainless steel bar is provided in the present disclosure, which is prepared by using the processing method described above.
- the present disclosure has at least the following beneficial effects.
- the processing method of the present disclosure can avoid the quality defect of surface crack while ensuring production efficiency by optimizing the process flow and controlling key process parameters, so that a niobium-containing high-alloy austenitic heat-resistant stainless steel bar with good surface quality and structure can be prepared.
- the bar was tested in accordance with the standard test method for determination of average grain size in ASTME112-2013.
- the grain size is in a range of Grade 3 to Grade 7, and the grade difference is less than 2.
- FIG. 1 shows a microstructure of a bar in Example 1 of the present disclosure.
- the inventors of the present disclosure conducted an in-depth analysis on the hot working process, and conducted an in-depth study on the process flow and key process parameters.
- the elemental composition (in wt%) of the niobium-containing high-alloy austenitic heat-resistant stainless steel is: C 0.04% to 0.10%, Si ⁇ 0.75%, Mn ⁇ 2.00%, P ⁇ 0.03%, S ⁇ 0.03%, Cr 24.0% to 26.0%, Ni 17.0% to 23.0%, Nb 0.20% to 0.60%, N 0.15% to 0.35%, and a balance of Fe and unavoidable impurities.
- the characteristics of the niobium-containing high-alloy austenitic heat-resistant stainless steel bar are as follows: (i) due to high alloying element contents, especially the high N content, the resistance to thermal deformation is relatively large, and the resistance to thermal deformation at a temperature of 1,100 ⁇ 1,200 °C is 1.5 ⁇ 2 times that of a conventional austenitic stainless steel; and (ii) the high Cr content and the inclusion of Nb lead to the easy formation of massive niobium compounds at places where Nb elements are concentrated during solidification of the alloy, resulting in local plastic deterioration and local cracks.
- the inventor of the present disclosure proposed a hot working method of a niobium-containing high-alloy austenitic stainless steel bar.
- the overall process route of the method is: steel ingot -> heating -> radical forging of square billet -> surface inspection and grinding -> heating -> hot rolling -> straightening -> inspection.
- the processing method of a stainless steel bar according to the present disclosure comprises:
- the steel ingot is heated at a heating temperature in a range from 1,200 °C to 1,270 °C, for example, 1,200 °C, 1,210 °C, 1,220 °C, 1,230 °C, 1,240 °C, 1,250 °C, 1,260 °C, or 1,270 °C, etc.
- a low melting point eutectic phase of niobium may be produced when the stainless steel was heated at a temperature greater than 1,270 °C
- a NbC phase may be produced when the stainless steel was heated at a temperature less than 1,200 °C.
- the selection of the target heating temperature in a range from 1,200 °C to 1,270 °C can not only protect the thermoplasticity from the production of the NbC phase, but also avoid the generation of low melting point eutectic phase at high temperatures.
- the holding time after the required temperature is achieved depends on the diameter D of the steel ingot, and usually is in a range from 0.5 to 0.8 min/mm.
- the steel ingot can be fully heated within this time period without NbC dissolved.
- the holding time should be appropriately extended.
- the holding time is too long, it will easily lead to too coarse grains and grain boundary cracks. Therefore, it is proposed to determine the holding time of the steel ingot by using the above equation. In this way, the massive NbC can be eliminated as much as possible, and at the same time the grain can be prevented from being too massive or grain boundary cracks can be avoided.
- the heated steel ingot is subjected to cogging by radial forging.
- the total compression ratio during the radial forging should be greater than 3, in order to ensure that the as-cast structure of the steel ingot has been broken.
- the surface temperature at the beginning of the radial forging should be controlled in a range from 1,000 °C to 1,050 °C, for example, 1,000 °C, 1,010 °C, 1,020 °C, 1,030 °C, 1,040 °C or 1,050 °C.
- the surface temperature during the radial forging and at the end of the radial forging should be controlled in a range from 900 °C to 1,050 °C, for example, 900 °C, 910 °C, 920 °C, 930 °C, 940 °C or 950 °C.
- the grain after dynamic recrystallization is prevented from being growing at low temperatures so as to ensure the fine grain structure of the surface.
- the radial forging is carried out in 4 to 10 passes, for example, 4 passes, 5 passes, 6 passes, 7 passes, 8 passes, 9 passes or 10 passes.
- high-frequency forging with small deformation is adopted, and the deformation for a single pass is in a range from 4% to 8%, such as 4%, 5%, 6%, 7% or 8%, and the forging frequency is in a range from 200 times/min to 240 times/min, such as 200 times/min, 210 times/min, 220 times/min, 230 times/min or 240 times/min.
- the as-cast structure of the surface is broken by rapid forging and small deformation, so that the surface becomes a recrystallized structure with uniform and fine grains and a surface hardening layer is formed, preventing cracks caused by poor plasticity of the cast billet during the cogging.
- low-frequency forging with large deformation is adapted.
- the low forging rate and large deformation are adapted to ensure adequate recrystallization in the core deformation process, improving the core structure.
- the deformation for a single pass is in a range from 15% to 20%, for example, 15%, 16%, 17%, 18%, 19% or 20%.
- the forging frequency is in a range from 30 times/min to 50 times/min, for example, 30 times/min, 31 times/min, 32 times/min, 33 times/min, 34 times/min, 35 times/min, 36 times/min, 37 times/min, 38 times/min, 39 times/min, 40 times/min, 41 times/min, 42 times/min, 43 times/min, 44 times/min, 45 times/min, 46 times/min, 47 times/min, 48 times/min, 49 times/min or 50 times/min.
- the resulting product is a square billet.
- the square billet is heated at a heating temperature in a range from 1,200 °C to 1,270 °C, for example, 1,200 °C, 1,210 °C, 1,220 °C, 1,230 °C, 1,240 °C, 1,250 °C, 1,260 °C or 1,270 °C.
- 0.5L is used as a minimum time to ensure uniform firing of the square billet.
- the holding time after the desired temperature is achieved is usually in a range from 0.5 min/mm to 0.8 min/mm, depending on the diameter D of the steel ingot. In the present disclosure, the holding time is extended to 1.0 min/mm in order to ensure effects.
- half of the side length is adopted for the time according to the inventor's study, and 0.5mm is taken as a factor, so that a uniform temperature inside and outside of the heated square billet can be ensured, which can reach 10 °C or lower.
- 100 ⁇ w(Nb) ⁇ 100 is taken as an additional minimum holding time to maximize the dissolution of NbC, reduce the segregation of other alloy elements, and improve the natural plasticity of the material.
- the hot rolling is mainly used to protect the surface from cracking.
- the surface temperature at the beginning of the hot rolling should be controlled in a range from 1,100°C to1,150 °C (for example, 1,100 °C, 1,110 °C, 1,120 °C, 1,130 °C, 1,140 °C or 1,150 °C). Since the thermoplastic of the stainless steel material drops sharply below 1,000 °C and it is easier to crack due to tensile stress during the hot rolling, the surface temperature during the hot rolling and at the end of the hot rolling should be controlled at 1,000 °C or more, and it is ensured that the temperature is in the optimal thermoplastic zone.
- the hot rolling is carried out in 5 ⁇ 10 passes, such as, 5 passes, 6 passes, 7 passes, 8 passes, 9 passes or 10 passes.
- the deformation for a single pass is in a range from 10% to 15%, such as 10%, 11%, 12%, 13%, 14% or 15%, and the rolling speed is in a range from 1 m/s to 1.2m/s, such as 1m/s, 1.1m/s or 1.2m/s.
- the hot rolling is mainly used for forming, that is, during the production, the stainless steel material is produced to a product with required specifications in the deformation temperature range (i.e., the best deformation temperature range), to avoid waste products caused by cracking due to the hot rolling.
- the elemental composition of stainless steel in this example is: C 0.05%, Si 0.23%, Mn 0.78%, P 0.021%, S 0.001%, Cr 25.1%, Ni 21.2%, Nb 0.5%, N 0.26%, and a balance of Fe.
- the steel ingot has a size of ⁇ 600mm, and the target bar has a size of ⁇ 140mm.
- the general process route comprises: steel ingot -> heating -> radial forging of square billet -> surface inspection and grinding -> heating -> hot rolling -> straightening -> inspection.
- the details are as follows:
- the microstructure of the prepared bar is shown in FIG. 1 . It can be seen from FIG. 1 that the bar possesses good structure and uniform grain size.
- the Nb content of a steel grade is 0.4%
- the steel ingot has a size of ⁇ 500mm
- the target bar has a size of ⁇ 140mm.
- the general process route comprises: steel ingot -> heating -> radial forging of square billet -> surface inspection and grinding -> heating -> hot rolling -> straightening -> inspection. The details are as follows:
Abstract
The present disclosure relates to a hot working method for stainless steel bars, comprising: step (1) heating a steel ingot at a heating temperature in a range from 1200 °C to 1270 °C; step (2) subjecting the heated steel ingot to radial forging and cogging down according to a total compression ratio greater than 3 to obtain a square billet; step (3) heating the square billet at a heating temperature in a range from 1200 °C to 1270 °C; and step (4) subjecting the heated square billet to hot rolling to obtain a bar. The processing method of the present disclosure can avoid the quality defect of surface crack while ensuring production efficiency by optimizing the process flow and controlling key process parameters, so that niobium-containing high-alloy austenitic heat-resistant stainless steel bars with good surface quality and structure can be prepared.
Description
- The present disclosure claims the priority of
Chinese Patent Application No. 202110795237.3, filed with the China National Intellectual Property Administration on July 14, 2021 - The present disclosure relates to the technical field of steel material processing, and in particular, to a hot working method of a niobium-containing high-alloy austenitic heat-resistant stainless steel bar.
- In order to improve power generation efficiency and reduce emissions, parameters in thermal power plants, such as steam temperatures and pressures, have been constantly increased. Traditional materials cannot meet the requirements of unit boilers, since the boiler superheater and reheater pipes, as the most demanding parts in the service environment, require use of a large number of a high-performance niobium-containing austenitic heat-resistant stainless steel material. The added Nb element can generate nano-sized MX and NbCrN during service, and the nano-sized MX and NbCrN can be well diffused in the matrix, impede dislocation movement, and improve generation strengthening effect and creep resistance, improving the high temperature resistance of austenitic stainless steel. However, due to the high contents of Cr and Ni in such material, it is easy to form massive niobium compounds during solidification, which significantly reduces the thermoplasticity of the material so that the material is very easy to crack during hot working.
- In recent years, in order to reduce production costs, pipe manufacturing enterprises in China have gradually begun to develop hot piercing processes to replace traditional hot extrusion processes, and the size of raw material bars is getting smaller and smaller, from the previous φ 180 ~ 250mm gradually reduced to φ 65 ~ 130mm. Originally, a preparation process of this kind of material bar is direct radial forging of a steel ingot in one heat forging. In the case where the size of a finished product is reduced, if the original process is used for direct forging for forming, repeated firing forging will be needed due to too many passes and too large temperature drop during the forging process, which renders production efficiency is too low; and if a stainless steel ingot is subjected to conventional cogging by primary rolling + hot rolling or cogging by primary rolling + forging to produce a bar with a small size, the surface cracking is serious.
- At present, there is an urgent need for a hot working method of a niobium-containing high-alloy austenitic heat-resistant stainless steel bar in order to solve these problems.
- In view of the above problems, the present disclosure is proposed in order to provide a hot working method of a niobium-containing high-alloy austenitic heat-resistant stainless steel bar that overcomes the above problems or at least partially solves the above problems.
- In one aspect, a hot working method for a stainless steel bar is provided in the present disclosure, comprising:
- Step (1): heating a steel ingot at a heating temperature in a range from 1,200 °C to 1,270 °C;
- Step (2): subjecting the heated steel ingot to cogging by radial forging at a total compression ratio greater than 3 to obtain a square billet;
- Step (3): heating the square billet at a heating temperature in a range from 1,200 °C to 1,270 °C; and
- Step (4) subjecting the heated square billet to hot rolling to obtain a bar.
-
- T1 is a holding time for the steel ingot, in minutes;
- D is a diameter of the steel ingot, in millimetres; and
- w(Nb) is a mass percent content of Nb.
- Optionally, in step (2), a surface temperature at the beginning of the radial forging is in a range from 1,000 °C to 1,050 °C, and a surface temperature during the radial forging and at the end of the radial forging is in a range from 900 °C to 950 °C.
- Optionally, in step (2), the radial forging is carried out in 4 to 10 passes, a single pass deformation and a forging frequency for the first and second passes are respectively 4% to 8% and 200 to 240 times/min, and a single pass deformation and a forging frequency for the subsequent passes are respectively 15% to 20% and 30 to 50 times/min.
-
- T2 is a holding time for the square billet, in minutes;
- L is a side length of the square billet, in millimetres; and
- w(Nb) is a mass percent content of Nb.
- Optionally, in step (4), a surface temperature at the beginning of the hot rolling is in a range from 1,100 °C to 1,150 °C, and a surface temperature during the hot rolling and at the end of the hot rolling 1,000 °C or more.
- Optionally, in step (4), the hot rolling is carried out in 5 to 10 passes, a single pass deformation is 10%~15%, and a rolling speed is 1~1.2m/s.
- Optionally, the stainless steel bar has the following elemental composition:
- C 0.04% to 0.10%, Si ≤ 0.75%, Mn ≤ 2.00%, P < 0.03%, S < 0.03%, Cr 24.0% to 26.0%, Ni 17.0% to 23.0%, Nb 0.20% to 0.60%, N 0.15% to 0.35%, and a balance of Fe.
- In another aspect, a stainless steel bar is provided in the present disclosure, which is prepared by using the processing method described above.
- Compared with the prior art, the present disclosure has at least the following beneficial effects.
- The processing method of the present disclosure can avoid the quality defect of surface crack while ensuring production efficiency by optimizing the process flow and controlling key process parameters, so that a niobium-containing high-alloy austenitic heat-resistant stainless steel bar with good surface quality and structure can be prepared.
- The bar was tested in accordance with the standard test method for determination of average grain size in ASTME112-2013. The grain size is in a range of Grade 3 to Grade 7, and the grade difference is less than 2.
-
FIG. 1 shows a microstructure of a bar in Example 1 of the present disclosure. - In order to fully understand the purpose, features and effects of the present disclosure, the present disclosure is described in detail through the following specific embodiments. Except for the following content, conventional methods or devices in the field are adopted in the process method of the present disclosure. The following terminologies have the meanings commonly understood by those skilled in the art, unless otherwise stated.
- In response to the technical difficulties encountered in the production of small-size niobium-containing high-alloy austenitic stainless steel bars, the inventors of the present disclosure conducted an in-depth analysis on the hot working process, and conducted an in-depth study on the process flow and key process parameters.
- The elemental composition (in wt%) of the niobium-containing high-alloy austenitic heat-resistant stainless steel is:
C 0.04% to 0.10%, Si ≤ 0.75%, Mn ≤ 2.00%, P < 0.03%, S < 0.03%, Cr 24.0% to 26.0%, Ni 17.0% to 23.0%, Nb 0.20% to 0.60%, N 0.15% to 0.35%, and a balance of Fe and unavoidable impurities. - It is found by the inventor through researches that the characteristics of the niobium-containing high-alloy austenitic heat-resistant stainless steel bar are as follows: (i) due to high alloying element contents, especially the high N content, the resistance to thermal deformation is relatively large, and the resistance to thermal deformation at a temperature of 1,100~1,200 °C is 1.5~2 times that of a conventional austenitic stainless steel; and (ii) the high Cr content and the inclusion of Nb lead to the easy formation of massive niobium compounds at places where Nb elements are concentrated during solidification of the alloy, resulting in local plastic deterioration and local cracks.
- Based on the above research findings, it is further believed that, in terms of process design, due to extremely poor thermoplasticity of the stainless steel material in the form of as-cast structure, if cogging by rolling is adopted, the rolled metal is subjected to tensile stress deformation, however, the deformation conditions are poor, which cannot give full play to the full plasticity of the metal, resulting in surface cracking. Therefore, cogging by radial forging is selected, and the metal is mainly subjected to compressive stress during the deformation process, which results in good plasticity. After radial forging, the deformation recrystallization structure is formed, and the thermoplasticity is greatly improved. In order to improve the production efficiency, the rolling process is used to mold the material. Therefore, the deformation is conducted through the process of "radial forging + rolling".
- Based on the above research findings and technical thinking, the inventor of the present disclosure proposed a hot working method of a niobium-containing high-alloy austenitic stainless steel bar. The overall process route of the method is: steel ingot -> heating -> radical forging of square billet -> surface inspection and grinding -> heating -> hot rolling -> straightening -> inspection.
- Specifically, the processing method of a stainless steel bar according to the present disclosure comprises:
-
- T1 is a holding time for the steel ingot, in minutes;
- D is a diameter of the steel ingot, in millimetres; and
- w(Nb) is a mass percent content of Nb.
- By analyzing the phase diagram of the above niobium-containing high-alloy austenitic stainless steel, supplemented by experimental observations, the inventors found that a low melting point eutectic phase of niobium may be produced when the stainless steel was heated at a temperature greater than 1,270 °C, and a NbC phase may be produced when the stainless steel was heated at a temperature less than 1,200 °C. However, due to the segregation of the Nb element during solidification, the NbC phase produced in the steel ingot is often massive. Therefore, the selection of the target heating temperature in a range from 1,200 °C to 1,270 °C can not only protect the thermoplasticity from the production of the NbC phase, but also avoid the generation of low melting point eutectic phase at high temperatures.
- In addition, it is also found through studies that, in a conventional heating system, the holding time after the required temperature is achieved depends on the diameter D of the steel ingot, and usually is in a range from 0.5 to 0.8 min/mm. The steel ingot can be fully heated within this time period without NbC dissolved. In order to eliminate the massive NbC as much as possible, the holding time should be appropriately extended. However, if the holding time is too long, it will easily lead to too coarse grains and grain boundary cracks. Therefore, it is proposed to determine the holding time of the steel ingot by using the above equation. In this way, the massive NbC can be eliminated as much as possible, and at the same time the grain can be prevented from being too massive or grain boundary cracks can be avoided.
- The heated steel ingot is subjected to cogging by radial forging. The total compression ratio during the radial forging should be greater than 3, in order to ensure that the as-cast structure of the steel ingot has been broken.
- The surface temperature at the beginning of the radial forging should be controlled in a range from 1,000 °C to 1,050 °C, for example, 1,000 °C, 1,010 °C, 1,020 °C, 1,030 °C, 1,040 °C or 1,050 °C. The surface temperature during the radial forging and at the end of the radial forging should be controlled in a range from 900 °C to 1,050 °C, for example, 900 °C, 910 °C, 920 °C, 930 °C, 940 °C or 950 °C. The grain after dynamic recrystallization is prevented from being growing at low temperatures so as to ensure the fine grain structure of the surface.
- The radial forging is carried out in 4 to 10 passes, for example, 4 passes, 5 passes, 6 passes, 7 passes, 8 passes, 9 passes or 10 passes. For the first two passes, high-frequency forging with small deformation is adopted, and the deformation for a single pass is in a range from 4% to 8%, such as 4%, 5%, 6%, 7% or 8%, and the forging frequency is in a range from 200 times/min to 240 times/min, such as 200 times/min, 210 times/min, 220 times/min, 230 times/min or 240 times/min. In this way, the as-cast structure of the surface is broken by rapid forging and small deformation, so that the surface becomes a recrystallized structure with uniform and fine grains and a surface hardening layer is formed, preventing cracks caused by poor plasticity of the cast billet during the cogging. For the subsequent passes, low-frequency forging with large deformation is adapted. The low forging rate and large deformation are adapted to ensure adequate recrystallization in the core deformation process, improving the core structure. The deformation for a single pass is in a range from 15% to 20%, for example, 15%, 16%, 17%, 18%, 19% or 20%. The forging frequency is in a range from 30 times/min to 50 times/min, for example, 30 times/min, 31 times/min, 32 times/min, 33 times/min, 34 times/min, 35 times/min, 36 times/min, 37 times/min, 38 times/min, 39 times/min, 40 times/min, 41 times/min, 42 times/min, 43 times/min, 44 times/min, 45 times/min, 46 times/min, 47 times/min, 48 times/min, 49 times/min or 50 times/min. The resulting product is a square billet.
-
- T2 is a holding time for the square billet, in minutes;
- L is a side length of the square billet, in millimetres; and
- w(Nb) is a mass percent content of Nb.
- In this equation, 0.5L is used as a minimum time to ensure uniform firing of the square billet. The holding time after the desired temperature is achieved is usually in a range from 0.5 min/mm to 0.8 min/mm, depending on the diameter D of the steel ingot. In the present disclosure, the holding time is extended to 1.0 min/mm in order to ensure effects. In actual production, considering that stacked square billets in one batch are heated from three sides, one of which is at the bottom side of the furnace, half of the side length is adopted for the time according to the inventor's study, and 0.5mm is taken as a factor, so that a uniform temperature inside and outside of the heated square billet can be ensured, which can reach 10 °C or lower. In order to maximize the dissolution of NbC, 100×w(Nb)×100 is taken as an additional minimum holding time to maximize the dissolution of NbC, reduce the segregation of other alloy elements, and improve the natural plasticity of the material.
- Since the as-cast structure has been broken during the radial forging in step (2), the hot rolling is mainly used to protect the surface from cracking. The surface temperature at the beginning of the hot rolling should be controlled in a range from 1,100°C to1,150 °C (for example, 1,100 °C, 1,110 °C, 1,120 °C, 1,130 °C, 1,140 °C or 1,150 °C). Since the thermoplastic of the stainless steel material drops sharply below 1,000 °C and it is easier to crack due to tensile stress during the hot rolling, the surface temperature during the hot rolling and at the end of the hot rolling should be controlled at 1,000 °C or more, and it is ensured that the temperature is in the optimal thermoplastic zone.
- Depending on finished product specifications, the hot rolling is carried out in 5~10 passes, such as, 5 passes, 6 passes, 7 passes, 8 passes, 9 passes or 10 passes. The deformation for a single pass is in a range from 10% to 15%, such as 10%, 11%, 12%, 13%, 14% or 15%, and the rolling speed is in a range from 1 m/s to 1.2m/s, such as 1m/s, 1.1m/s or 1.2m/s.
- Then, straightening and inspection can be carried out by conventional methods, which will not be described here.
- The hot rolling is mainly used for forming, that is, during the production, the stainless steel material is produced to a product with required specifications in the deformation temperature range (i.e., the best deformation temperature range), to avoid waste products caused by cracking due to the hot rolling.
- The present disclosure is further described by way of embodiments, however, it is not thereby limited to the described embodiments. Experimental methods for which specific conditions are not indicated in the following embodiments are selected according to conventional methods and conditions, or according to the product description.
- The elemental composition of stainless steel in this example is: C 0.05%, Si 0.23%, Mn 0.78%, P 0.021%, S 0.001%, Cr 25.1%, Ni 21.2%, Nb 0.5%, N 0.26%, and a balance of Fe.
- The steel ingot has a size of Φ600mm, and the target bar has a size of Φ140mm.
- The general process route comprises: steel ingot -> heating -> radial forging of square billet -> surface inspection and grinding -> heating -> hot rolling -> straightening -> inspection. The details are as follows:
- 1. Heating of steel ingot: the target temperature is 1,230 °C, and the holding time is 600 min.
- 2. Cogging by radial forging: the temperature at the beginning of the radial forging is 1,030 °C; the radial forging is carried out in 9 passes, and for the first 2 passes, the deformation is 6%, and the radial forging frequency is 210 times/min; and for the last 7 passes, the deformation of is 16%, and the radial forging frequency is 45 times/min. The temperature at the end of the radial forging is 920 °C, and the square billet has a size of 271 × 271 mm.
- 3. Heating of square billet: the heating temperature is 1,250 °C, and the holding time is 185 min.
- 4. Hot rolling: the surface temperature at the beginning of the hot rolling is 1,120 °C, and the surface temperature at the end of the hot rolling is 1,050 °C. The hot rolling is carried out in 8 passes. The deformation for the first 4 passes is 13%, the deformation for the last 4 passes is 11%, and the rolling speed is 1 m/s.
- The microstructure of the prepared bar is shown in
FIG. 1 . It can be seen fromFIG. 1 that the bar possesses good structure and uniform grain size. - The Nb content of a steel grade is 0.4%, the steel ingot has a size of φ 500mm, and the target bar has a size of φ 140mm. The general process route comprises: steel ingot -> heating -> radial forging of square billet -> surface inspection and grinding -> heating -> hot rolling -> straightening -> inspection. The details are as follows:
- 1. Heating of steel ingot: the target temperature is 1,230 °C, and the holding time is 490 min.
- 2. Cogging by radial forging: the temperature at the beginning of the radial forging is 1,040 °C; the forging is carried out in 9 passes, and for the first 2 passes, the deformation is 7%, and the radial forging frequency is 215 times/min; and for the last 7 passes, the deformation of is 20%, and the radial forging frequency is 50 times/min. The temperature at the end of the radial forging is 930 °C, and the square billet has a size of 220 × 220mm.
- 3. Heating of square billet: the heating temperature is 1,260 °C, and the holding time is 150 min.
- 4. Hot rolling: the surface temperature at the beginning of the hot rolling is 1,130 °C, and the surface temperature at the end of the hot rolling is 1,030 °C. The rolling is carried out in 7 passes. The deformation for the first 5 passes is 15%, the deformation for the last 2 passes is 13%, and the rolling speed is 1.1 m/s.
- According to the standard test method for determination of average grain size in ASTME112-2013, the bars obtained in Examples 1 and 2 were tested for tissue grain size, and the results were as follows.
Table 1 Grain size Range Example 1 5-6 1 Example 2 4-5 1 - The above-mentioned examples are preferred examples of the present disclosure. However, the embodiments of the present disclosure are not limited by the above-mentioned examples, and any other substitutions, modifications, combinations, changes, modifications, and simplifications made without departing from the spirit and principle of the present disclosure should all be equivalent replacement methods, which are all included in the protection scope of the present disclosure.
Claims (9)
- A hot working method of a stainless steel bar, comprising:Step (1): heating a steel ingot at a heating temperature in a range from 1,200 °C to 1,270 °C;Step (2): subjecting the heated steel ingot to cogging by radial forging at a total compression ratio greater than 3 to obtain a square billet;Step (3): heating the square billet at a heating temperature in a range from 1,200 °C to 1,270 °C; andStep (4) subjecting the heated square billet to hot rolling to obtain a bar.
- The method according to claim 1, wherein in step (2), a surface temperature at the beginning of the radial forging is in a range from 1,000 °C to 1,050 °C, and a surface temperature during the radial forging and at the end of the radial forging is in a range from 900 °C to 950 °C.
- The method according to claim 1, wherein in step (2), the radial forging is carried out in 4 to 10 passes, a single pass deformation and a forging frequency for first and second passes are respectively 4% to 8% and 200 to 240 times/min, and a single pass deformation and a forging frequency for the subsequent passes are respectively 15% to 20% and 30 to 50 times/min.
- The method according to claim 1, wherein in step (4), a surface temperature at the beginning of the hot rolling is in a range from 1,100 °C to 1,150 °C, and a surface temperature during the hot rolling and at the end of the hot rolling is 1,000 °C or more.
- The method according to claim 1, wherein in step (4), the hot rolling is carried out in 5 to 10 passes, a single pass deformation is 10%~15%, and a rolling speed is 1~1.2m/s.
- The method according to claim 1, wherein the stainless steel bar has the following elemental composition:
C 0.04% to 0.10%, Si ≤ 0.75%, Mn ≤ 2.00%, P < 0.03%, S < 0.03%, Cr 24.0% to 26.0%, Ni 17.0% to 23.0%, Nb 0.20% to 0.60%, N 0.15% to 0.35%, and a balance of Fe. - A stainless steel bar, which is prepared by using the hot working method in any one of claims 1 to 8.
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